U.S. patent application number 15/328150 was filed with the patent office on 2017-07-27 for fuel cell system and control method for fuel cell system.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. The applicant listed for this patent is NISSAN MOTOR CO., LTD.. Invention is credited to Kiyoshi HOSHI.
Application Number | 20170214069 15/328150 |
Document ID | / |
Family ID | 55162868 |
Filed Date | 2017-07-27 |
United States Patent
Application |
20170214069 |
Kind Code |
A1 |
HOSHI; Kiyoshi |
July 27, 2017 |
FUEL CELL SYSTEM AND CONTROL METHOD FOR FUEL CELL SYSTEM
Abstract
A fuel cell system includes a gas supply passage configured to
supply one of the anode gas and the cathode gas to the fuel cell, a
refrigerant supply apparatus that supplies refrigerant for cooling
the fuel cell to the fuel cell, a heat exchanger that exchanges
heat between the refrigerant increased in temperature by the fuel
cell and the gas supplied to the gas supply passage. The fuel cell
includes a component that circulates the one of the anode gas and
the cathode gas discharged from the fuel cell to the fuel cell, and
a warm-up control unit that controls a flow rate of the refrigerant
to a predetermined flow rate for warming up the fuel cell when the
fuel cell is warmed up. The fuel cell system includes a gas
temperature increase control unit increases the flow rate of the
refrigerant to be supplied to the heat exchanger on the basis of a
temperature of the gas circulated by the component or a parameter
related to the temperature when the flow rate of the refrigerant is
controlled by the warm-up control unit.
Inventors: |
HOSHI; Kiyoshi; (Kanagawa,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NISSAN MOTOR CO., LTD. |
Yokohama-shi, Kanagawa |
|
JP |
|
|
Assignee: |
NISSAN MOTOR CO., LTD.
Yokohama-shi, Kanagawa
JP
|
Family ID: |
55162868 |
Appl. No.: |
15/328150 |
Filed: |
June 16, 2015 |
PCT Filed: |
June 16, 2015 |
PCT NO: |
PCT/JP2015/067336 |
371 Date: |
January 23, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/04731 20130101;
H01M 8/04328 20130101; H01M 8/04253 20130101; H01M 8/04708
20130101; H01M 8/04029 20130101; H01M 8/04225 20160201; H01M
8/04014 20130101; H01M 8/04097 20130101; H01M 8/04768 20130101;
H01M 8/0485 20130101; H01M 8/04067 20130101; Y02E 60/50 20130101;
H01M 2008/1095 20130101; H01M 8/04343 20130101; H01M 8/04268
20130101; H01M 8/04302 20160201 |
International
Class: |
H01M 8/04746 20060101
H01M008/04746; H01M 8/0432 20060101 H01M008/0432; H01M 8/04029
20060101 H01M008/04029; H01M 8/04701 20060101 H01M008/04701; H01M
8/04828 20060101 H01M008/04828 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 24, 2014 |
JP |
2014-151268 |
Claims
1.-10. (canceled)
11. A fuel cell system for supplying anode gas and cathode gas to a
fuel cell and causing the fuel cell to generate power according to
a load, comprising: a gas supply passage configured to supply one
of the anode gas and the cathode gas to the fuel cell; a
refrigerant supply apparatus configured to supply refrigerant for
cooling the fuel cell to the fuel cell; a heat exchanger configured
to exchange heat between the refrigerant increased in temperature
by the fuel cell and the gas supplied to the gas supply passage; a
component provided in the gas supply passage and configured to
circulate the one of the anode gas and the cathode gas discharged
from the fuel cell to the fuel cell; a warm-up control unit
configured to control a flow rate of the refrigerant to a
predetermined flow rate for warming up the fuel cell when the fuel
cell is warmed up; and a gas temperature increase control unit
configured to increase the flow rate of the refrigerant to be
supplied to the heat exchanger on the basis of a temperature of
circulation gas circulated by the component or a parameter related
to the temperature and a temperature of supplied gas before joining
with the circulation gas when the flow rate of the refrigerant is
controlled by the warm-up control unit.
12. The fuel cell system according to claim 11, wherein: the gas
temperature increase control unit increases the flow rate of the
refrigerant when a temperature of circulating gas circulated from
the fuel cell to the component is not lower than a freezing point
temperature and a temperature of discharged gas discharged from the
component to the fuel cell is not higher than the freezing point
temperature.
13. The fuel cell system according to claim 11, wherein: the gas
temperature increase control unit increases the flow rate of the
refrigerant to be higher than the flow rate controlled by the
warm-up control unit when a temperature of circulating gas
circulated from the fuel cell to the component exceeds a
predetermined threshold value beyond which the amount of steam in
the circulating gas increases.
14. The fuel cell system according to claim 13, wherein: the gas
temperature increase control unit increases an increase width of
the flow rate of the refrigerant as a temperature difference
increases, the temperature difference being a difference between a
temperature of discharged gas discharged from the component to the
fuel cell and a temperature of circulating gas.
15. The fuel cell system according to claim 13, wherein: the gas
temperature increase control unit increases an increase width of
the flow rate of the refrigerant as a supply flow rate of the gas
to be supplied to the fuel cell increases.
16. The fuel cell system according to claim 13, wherein: the gas
temperature increase control unit decreases an increase width of
the flow rate of the refrigerant as an electrolyte membrane of the
fuel cell becomes drier.
17. The fuel cell system according to claim 11, wherein: the gas
temperature increase control unit limits an increase of the flow
rate of the refrigerant to be supplied to the heat exchanger on the
basis of a temperature of discharged gas discharged from the
component to the fuel cell.
18. The fuel cell system according to claim 17, further comprising:
a calculation unit configured to calculate the temperature of the
discharged gas on the basis of a temperature of supplied gas
supplied from the heat exchanger to the component and a temperature
of circulating gas circulated from the fuel cell to the component,
wherein: the gas temperature increase control unit switches the
flow rate of the refrigerant to be supplied to the heat exchanger
to the flow rate controlled by the warm-up control unit when the
temperature of the discharged gas increases to a limit threshold
value determined on the basis of a freezing point temperature after
the flow rate of the refrigerant is increased.
19. The fuel cell system according to claim 11, wherein: the
parameter related to the temperature includes a temperature of the
refrigerant.
20. A control method for a fuel cell system for supplying anode gas
and cathode gas to a fuel cell and causing the fuel cell to
generate power according to a load, the fuel cell system including
a gas supply passage configured to supply one of the anode gas and
the cathode gas to the fuel cell, a refrigerant supply apparatus
configured to supply refrigerant for cooling the fuel cell to the
fuel cell, a heat exchanger configured to exchange heat between the
refrigerant increased in temperature by the fuel cell and the gas
supplied to the gas supply passage, and a component provided in the
gas supply passage and configured to circulate the one gas
discharged from the fuel cell to the fuel cell, the control method
comprising: a warm-up control step of controlling the flow rate of
the refrigerant to a predetermined flow rate for warming up the
fuel cell when the fuel cell is warmed up; and a gas temperature
increase control step of increasing the flow rate of the
refrigerant to be supplied to the heat exchanger on the basis of a
temperature of circulation gas circulated by the component or a
parameter related to the temperature and a temperature of supplied
gas before joining the circulation gas when the flow rate of the
refrigerant is controlled by the warm-up control step.
Description
TECHNICAL FIELD
[0001] This invention relates to a fuel cell system for circulating
anode gas discharged from a fuel cell to the fuel cell and a
control method for fuel cell system.
BACKGROUND ART
[0002] JP2010-146751A discloses a fuel cell system with a heat
exchanger configured to heat anode gas to be supplied to a fuel
cell, utilizing cooling water increased in temperature by the fuel
cell.
SUMMARY OF INVENTION
[0003] In a fuel cell system as described above, it is desirable to
reduce a flow rate of cooling water circulated to a fuel cell to
complete the warm-up of the fuel cell early when the fuel cell
system is started in a sub-zero temperature environment.
[0004] However, if the flow rate of the cooling water is reduced,
the amount of heat radiated to anode gas from the cooling water
increased in temperature by the fuel cell decreases in a heat
exchanger, wherefore a temperature increasing rate of the anode gas
is slowed.
[0005] During sub-zero start, the temperature of the anode gas
supplied from a tank may become lower than a freezing point and
steam in anode off-gas may be frozen to form ice in a flow passage
when the anode gas supplied from the tank and the anode off-gas
discharged from the fuel cell join.
[0006] When the flow rate of the cooling water is reduced as
described above in such a situation, the temperature increasing
rate of the anode gas is slowed. Thus, ice formed in the flow
passage increases and the flow passage may be closed.
[0007] The present invention was developed, focusing on such a
problem, and aims to provide a fuel cell system for preventing the
freezing of a component for circulating gas discharged from a fuel
cell to the fuel cell while realizing early warm-up of the fuel
cell and a control method for fuel cell system.
[0008] According to one aspect of the present invention, a fuel
cell system supplies anode gas and cathode gas to a fuel cell and
causes the fuel cell to generate power according to a load. The
fuel cell system includes a gas supply passage configured to supply
one of the anode gas and the cathode gas to the fuel cell, a
refrigerant supply apparatus configured to supply refrigerant for
cooling the fuel cell to the fuel cell, and a heat exchanger
configured to exchange heat between the refrigerant increased in
temperature by the fuel cell and the gas supplied to the gas supply
passage. The fuel cell system includes a component provided in the
gas supply passage and configured to circulate the one of the anode
gas and the cathode gas discharged from the fuel cell to the fuel
cell, and a warm-up control unit configured to control a flow rate
of the refrigerant to a predetermined flow rate for warming up the
fuel cell when the fuel cell is warmed up. The fuel cell system
includes a gas temperature increase control unit configured to
increase the flow rate of the refrigerant to be supplied to the
heat exchanger on the basis of a temperature of the gas circulated
by the component or a parameter related to the temperature when the
flow rate of the refrigerant is controlled by the warm-up control
unit.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a diagram showing the configuration of a fuel cell
system in a first embodiment of the present invention,
[0010] FIG. 2 is a block diagram showing a basic configuration of a
controller configured to control the fuel cell system,
[0011] FIG. 3 is a flow chart showing an example of a control
method for fuel cell system in the first embodiment,
[0012] FIG. 4 is a block diagram showing a functional configuration
for calculating a temperature of anode gas discharged from a jet
pump in the controller,
[0013] FIG. 5 is a block diagram showing the configuration of a
cooling water flow rate control unit in a second embodiment of the
present invention,
[0014] FIG. 6 is a graph showing a freezing prevention control map
determined to prevent the freezing of a gas flow passage,
[0015] FIG. 7 is a graph showing a correction map for correcting a
cooling water flow rate obtained by the freezing prevention control
map,
[0016] FIG. 8 is a graph showing an excessive temperature increase
prevention map for preventing the temperature of a fuel cell stack
from excessively increasing during a start-up processing of the
fuel cell system,
[0017] FIG. 9 are time charts showing a control technique of the
cooling water flow rate in the second embodiment,
[0018] FIG. 10 are time charts showing the control technique of the
cooling water flow rate when a temperature difference between
supplied gas to be supplied to a jet pump and circulating gas
becomes small,
[0019] FIG. 11 is a diagram showing the configuration of a fuel
cell system in a third embodiment of the present invention,
[0020] FIG. 12 is a diagram showing the configuration of a cooling
water flow rate control unit in the third embodiment,
[0021] FIG. 13 is a graph showing a rotation speed command map of a
cooling water pump,
[0022] FIG. 14 is a graph showing a rotation speed command map of a
bypass cooling water pump,
[0023] FIG. 15 is a diagram showing the configuration of a fuel
cell system in a fourth embodiment of the present invention,
[0024] FIG. 16 is a diagram showing the configuration of a cooling
water flow rate control unit in the fourth embodiment, and
[0025] FIG. 17 is a graph showing a bypass valve opening degree
command map.
DESCRIPTION OF EMBODIMENTS
[0026] Hereinafter, embodiments of the present invention are
described with reference to the accompanying drawings.
First Embodiment
[0027] FIG. 1 is a diagram showing a configuration example of a
fuel cell system in an embodiment of the present invention.
[0028] A fuel cell system 100 constitutes a power supply system for
supplying fuel gas necessary for power generation from outside to a
fuel cell and causing the fuel cell to generate power according to
an electric load. The fuel cell system 100 is controlled by a
controller 110.
[0029] The fuel cell system 100 includes a fuel cell stack 1, a
battery 2, a DC/DC converter 3, electric loads 4, a cathode gas
supplying/discharging device 10, an anode gas supplying/discharging
device 20, a stack cooling device 30 and a stack resistance
measuring device 45. Each of the cathode gas supplying/discharging
device 10, the anode gas supplying/discharging device 20 and the
stack cooling device 30 is an auxiliary machine for causing the
fuel cell stack 1 to generate power.
[0030] The battery 2 is a power supply for assisting the fuel cell
stack 1. The battery 2 outputs a voltage of, e.g. several hundreds
of V.
[0031] The DC/DC converter 3 is a bidirectional voltage converter
for adjusting a voltage of the fuel cell stack 1 and a voltage of
the battery 2 with respect to each other. The DC/DC converter 3 is
connected between the fuel cell stack 1 and the battery 2.
[0032] The DC/DC converter 3 is controlled by the controller 110
and adjusts the voltage of the fuel cell stack 1 using power output
from the battery 2. For example, the DC/DC converter 3 reduces the
voltage of the fuel cell stack 1 such that an output current taken
out from the fuel cell stack 1 increases as required power required
from the electric loads 4 increases.
[0033] The electric loads 4 are driven by power supplied from the
fuel cell stack 1 and the battery 2. Examples of the electric loads
4 include an electric motor for driving a vehicle and some of
auxiliary machines of the fuel cell stack 1.
[0034] In the present embodiment, the electric loads 4 are
connected to a power supply line connecting the fuel cell stack 1
and the DC/DC converter 3. It should be noted that the electric
motor may be connected to the power supply line between the fuel
cell stack 1 and the DC/DC converter 3 and some of the auxiliary
machines may be connected to the power supply line between the
battery 2 and the DC/DC converter 3.
[0035] The fuel cell stack 1 is such that several hundreds of
battery cells are laminated, and generates a DC voltage of, e.g.
several hundreds of V (volts).
[0036] A fuel cell is composed of an anode electrode (fuel
electrode), a cathode electrode (oxidant electrode) and an
electrolyte membrane sandwiched between the anode electrode and the
cathode electrode. In the fuel cell, anode gas (fuel gas)
containing hydrogen in the anode electrode and cathode gas (oxidant
gas) containing oxygen in the cathode electrode induce an
electrochemical reaction (power generation reaction) in the
electrolyte membrane. Specifically, the following electrochemical
reactions proceed in both anode and cathode electrodes.
Anode electrode: 2H.sub.2.fwdarw.4H.sup.++4e.sup.- (1)
Cathode electrode: 4H.sup.++4e.sup.-+O.sub.2.fwdarw.2H.sub.2P
(2)
[0037] By the above electrochemical reactions (1) and (2), an
electromotive force is generated and water is generated. Since each
of the fuel cells laminated in the fuel cell stack 1 is connected
in series, a total of cell voltages generated in the fuel cells
becomes an output voltage of the fuel cell stack 1.
[0038] The cathode gas is supplied to the fuel cell stack 1 from
the cathode gas supplying/discharging device 10 and the anode gas
is supplied thereto from the anode gas supplying/discharging device
20.
[0039] The cathode gas supplying/discharging device 10 is a device
configured to supply the cathode gas to the fuel cell stack 1 and
discharge cathode off-gas discharged from the fuel cell stack 1 to
atmosphere. The cathode off-gas contains excess cathode gas not
consumed by the fuel cell stack 1 and impurities such as generated
water associated with power generation.
[0040] The cathode gas supplying/discharging device 10 includes a
cathode gas supply passage 11, a compressor 12, a cathode gas
discharge passage 13, a cathode pressure control value 14, a bypass
passage 15 and a bypass valve 16.
[0041] The cathode gas supply passage 11 is a passage for supplying
the cathode gas to the fuel cell stack 1. One end of the cathode
gas supply passage 11 communicates with a passage for taking in air
containing oxygen from outside air and the other end is connected
to a cathode gas inlet hole of the fuel cell stack 1.
[0042] The compressor 12 is provided in the cathode gas supply
passage 11. The compressor 12 takes air into the cathode gas supply
passage 11 from outside air and supplies the air as the cathode gas
to the fuel cell stack 1. The compressor 12 is controlled by the
controller 110.
[0043] The cathode gas discharge passage 13 is a passage for
discharging the cathode off-gas from the fuel cell stack 1. One end
of the cathode gas discharge passage 13 is connected to a cathode
gas outlet hole of the fuel cell stack 1 and the other end is
open.
[0044] The cathode pressure control value 14 is provided in the
cathode gas discharge passage 13. In the present embodiment, an
electromagnetic valve capable of changing a valve opening degree in
a stepwise manner is used as the cathode pressure control valve 14.
The cathode pressure control value 14 is controlled to open and
close by the controller 110. By this open/close control, a pressure
of the cathode gas to be supplied to the fuel cell stack 1 is
adjusted to a desired pressure.
[0045] The bypass passage 15 is a passage for directly discharging
part of the cathode gas discharged from the compressor 12 to the
cathode gas discharge passage 13 without supplying it to the fuel
cell stack 1.
[0046] One end of the bypass passage 15 is connected to a part of
the cathode gas supply passage 11 between the compressor 12 and the
fuel cell stack 1 and the other end is connected to a part of the
cathode gas discharge passage 13 upstream of the cathode pressure
control value 14. Specifically, the bypass passage 15 is branched
off from the cathode gas supply passage 11 at a position downstream
of the compressor 12 and joins the cathode gas discharge passage 13
at a position upstream of the cathode pressure control valve
14.
[0047] The bypass valve 16 is provided in the bypass passage 15. In
the present embodiment, an electromagnetic valve capable of
changing a valve opening degree in a stepwise manner is used as the
bypass valve 16. The bypass valve 16 is controlled by the
controller 110.
[0048] The bypass valve 16 is opened, for example, when a flow rate
of the cathode gas necessary to dilute hydrogen discharged from the
fuel cell stack 1 (hereinafter, referred to as a "hydrogen dilution
request flow rate") becomes larger than a flow rate of the cathode
gas necessary for the fuel cell stack 1.
[0049] Alternatively, the bypass valve 16 is opened when a flow
rate of the cathode gas necessary to avoid a surge occurring in the
compressor 12 (hereinafter, referred to as a "surge avoidance
request flow rate") becomes larger than the flow rate of the
cathode gas necessary for the fuel cell stack 1.
[0050] It should be noted that the bypass valve 16 is closed when
the flow rate of the cathode gas necessary for the fuel cell stack
1 is larger than the hydrogen dilution request flow rate, the surge
avoidance request flow rate and the like.
[0051] The anode gas supplying/discharging device 20 supplies the
anode gas to the fuel cell stack 1 and removes impurities in anode
off-gas discharged from the fuel cell stack 1 while circulating the
anode off-gas to the fuel cell stack 1. The impurities mean
nitrogen in air permeating from the cathode electrodes to the anode
electrodes via the electrolyte membranes, generated water
associated with power generation and the like.
[0052] The anode gas supplying/discharging device 20 includes a
high-pressure tank 21, an anode gas supply passage 22, a heat
exchanger 23, an anode pressure control value 24, a jet pump 25, an
anode gas circulation passage 26, a gas-liquid separation device
27, a purge passage 28 and a purge valve 29.
[0053] The high-pressure tank 21 stores the anode gas to be
supplied to the fuel cell stack 1 in a high-pressure state.
[0054] The anode gas supply passage 22 is a passage for supplying
the anode gas stored in the high-pressure tank 21 to the fuel cell
stack 1. One end of the anode gas supply passage 22 is connected to
the high-pressure tank 21 and the other end is connected to an
anode gas inlet hole of the fuel cell stack 1.
[0055] The heat exchanger 23 is provided upstream of the anode
pressure control valve 24 in the anode gas supply passage 22. The
heat exchanger 23 exchanges heat between cooling water increased in
temperature in the fuel cell stack 1 and the anode gas supplied
from the high-pressure tank 21. The cooling water is refrigerant
for cooling the fuel cell stack 1.
[0056] When the fuel cell system 100 is started at a low
temperature, the heat exchanger 23 has a function of heating the
anode gas to be supplied to the anode gas supply passage 22 by the
cooling water warmed in the fuel cell stack 1.
[0057] The anode pressure control value 24 is provided between the
heat exchanger 23 and the jet pump 25 in the anode gas supply
passage 22. In the present embodiment, an electromagnetic valve
capable of changing a valve opening degree in a stepwise manner is
used as the anode pressure control valve 24. The anode pressure
control value 24 is controlled to open and close by the controller
110. By this open/close control, a pressure of the anode gas to be
supplied to the fuel cell stack 1 is adjusted.
[0058] A temperature sensor 41 configured to detect a temperature
of the anode gas supplied from the high-pressure tank 21
(hereinafter, referred to as a "supplied gas temperature") is
provided between the anode pressure control valve 24 and the jet
pump 25 in the anode gas supply passage 22. The temperature sensor
41 supplies a detection signal indicating the detected temperature
to the controller 110.
[0059] It should be noted that although the temperature sensor 41
is provided between the anode pressure control valve 24 and the jet
pump 25 in the anode gas supply passage 22 in the present
embodiment, it may be provided between the heat exchanger 23 and
the anode pressure control valve 24 in the anode gas supply passage
22.
[0060] The jet pump 25 is provided between the anode pressure
control valve 24 and the fuel cell stack 1 in the anode gas supply
passage 22. The jet pump 25 is a pump or ejector for causing the
anode gas circulation passage 26 to join the anode gas supply
passage 22. By using the jet pump 25, the anode off-gas can be
circulated to the fuel cell stack 1 by a simple configuration.
[0061] The jet pump 25 sucks the anode off-gas discharged from the
fuel cell stack 1 and circulates that anode off-gas to the fuel
cell stack 1 by increasing a flow velocity of the anode gas
supplied by the anode pressure control valve 24.
[0062] The jet pump 25 is composed, for example, of a nozzle and a
diffuser. The nozzle is for accelerating the flow velocity of the
anode gas and injecting the anode gas to the diffuser. The nozzle
is formed into a hollow cylindrical shape and an opening is
narrowed toward a tip part of the nozzle. In this way, the flow
velocity of the anode gas is increased in the tip part and the
anode gas is injected into the diffuser.
[0063] The diffuser is for sucking the anode off-gas by the flow
velocity of the anode gas injected from the nozzle. The diffuser
causes the anode gas injected from the nozzle and the sucked anode
off-gas to join and discharges gas after joining to the fuel cell
stack 1.
[0064] The diffuser is formed with a confluent passage on the same
axis as the nozzle. An opening of the confluent passage is formed
to be wider toward a discharge port. The diffuser is formed with a
hollow cylindrical suction chamber extending from a suction port to
the tip part of the nozzle and the suction chamber and the
confluent passage communicate.
[0065] A pressure sensor 42 is provided between the jet pump 25 and
the fuel cell stack 1 in the anode gas supply passage 22. The
pressure sensor 42 detects a pressure of the anode gas to be
supplied to the fuel cell stack 1 (hereinafter, referred to as a
"stack inlet gas pressure"). The pressure sensor 42 outputs a
detection signal indicating the detected pressure to the controller
110.
[0066] The anode gas circulation passage 26 is a passage for
circulating the anode off-gas discharged from the fuel cell stack 1
to the anode gas supply passage 22. One end of the anode gas
circulation passage 26 is connected to an anode gas outlet hole of
the fuel cell stack 1 and the other end joins a circulation port of
the jet pump 25.
[0067] The liquid-gas separation device 27 is provided in the anode
gas circulation passage 26. The liquid-gas separation device 27
separates impurities such as generated water and nitrogen gas in
the anode off-gas from excess anode gas. The liquid-gas separation
device 27 condenses steam contained in the anode off-gas into
liquid water.
[0068] The anode gas having the impurities removed in the
liquid-gas separation device 27 passes in the anode gas circulation
passage 26 and is supplied to the anode gas supply passage 22 again
via the jet pump 25. Further, a discharge hole for discharging the
impurities to the purge passage 28 is formed in a lower part of the
liquid-gas separation device 27.
[0069] The purge passage 28 is a passage for discharging the
impurities separated by the liquid-gas separation device 27. One
end of the purge passage 28 is connected to the discharge hole of
the liquid-gas separation device 27 and the other end is connected
to a part of the cathode gas discharge passage 13 downstream of the
cathode pressure control valve 14.
[0070] The purge valve 29 is provided in the purge passage 28. The
purge valve 29 is controlled to open and close by the controller
110. By this open/close control, the impurities such as nitrogen
gas and liquid water are discharged to the cathode gas discharge
passage 13.
[0071] The stack cooling device 30 is a device configured to adjust
the fuel cell stack 1 to a temperature suitable for power
generation, using the cooling water as refrigerant. The stack
cooling device 30 includes a cooling water circulation passage 31,
a cooling water pump 32, a radiator 33, a bypass passage 34, a
hater 35, a thermostat 36, a branch passage 37, a stack inlet water
temperature sensor 43 and a stack outlet water temperature sensor
44.
[0072] The cooling water circulation passage 31 is a passage for
circulating the cooling water to the fuel cell stack 1. One end of
the cooling water circulation passage 31 is connected to a cooling
water inlet hole of the fuel cell stack 1 and the other end is
connected to a cooling water outlet hole of the fuel cell stack
1.
[0073] The cooling water pump 32 is provided in the cooling water
circulation passage 31. The cooling water pump 32 constitutes a
refrigerant supply apparatus "refrigerant supply means" configured
to supply the cooling water to the fuel cell stack 1. The cooling
water pump 32 is controlled by the controller 110. It should be
noted that, without limitation to the cooling water pump, a
compressor may be used as the refrigerant supply apparatus
configured to supply the cooling water to the fuel cell stack
1.
[0074] The radiator 33 is provided on the side of a cooling water
suction port of the cooling water pump 32 in the cooling water
circulation passage 31. The radiator 33 cools the cooling water
heated by the fuel cell stack 1.
[0075] The bypass passage 34 is a passage bypassing the radiator
33. One end of the bypass passage 34 is connected to the cooling
water circulation passage 31 on a cooling water outlet side of the
fuel cell stack 1, and the other end is connected to the thermostat
36.
[0076] The heater 35 is provided in the bypass passage 34. The
heater 35 is energized to heat the cooling water when the fuel cell
stack 1 is warmed up. In the present embodiment, the heater 35
generates heat by having power supplied from the fuel cell stack 1
by the DC/DC converter 3.
[0077] The thermostat 36 is provided in a part where the bypass
passage 34 joins the cooling water circulation passage 31. The
thermostat 36 is a three-way valve. The thermostat 36 automatically
opens and closes in response to the temperature of the cooling
water flowing inside the thermostat 36.
[0078] For example, the thermostat 36 is closed and supplies only
the cooling water flowing by way of the bypass passage 34 to the
fuel cell stack 1 when the temperature of the cooling water is
lower than a predetermined valve opening temperature. In this way,
the cooling water having a higher temperature than the cooling
water flowing by way of the radiator 33 flows into the fuel cell
stack 1.
[0079] On the other hand, the thermostat 36 starts gradually
opening when the temperature of the cooling water becomes equal to
or higher than the valve opening temperature. Then, the thermostat
36 mixes the cooling water flowing by way of the bypass passage 34
and the cooling water flowing by way of the radiator 33 and
supplies the mixed cooling water to the fuel cell stack 1. In this
way, the cooling water having a lower temperature than the cooling
water flowing by way of the bypass passage 34 flows into the fuel
cell stack 1.
[0080] The branch passage 37 is branched off from the cooling water
circulation passage 31 between the cooling water pump 32 and the
cooling water inlet hole of the fuel cell stack 1, passes through
the heat exchanger 23 and joins the cooling water circulation
passage 31 at a position upstream of the bypass passage 34.
[0081] The stack inlet water temperature sensor 43 is provided near
the cooling water inlet hole of the fuel cell stack 1 in the
cooling water circulation passage 31. The stack inlet water
temperature sensor 43 detects a temperature of the cooling water
flowing into the fuel cell stack 1 (hereinafter, referred to as a
"stack inlet water temperature"). The stack inlet water temperature
sensor 43 outputs a detection signal indicating the detected
temperature to the controller 110.
[0082] The stack outlet water temperature sensor 44 is provided
near the cooling water outlet hole of the fuel cell stack 1 in the
cooling water circulation passage 31. The stack outlet water
temperature sensor 44 detects a temperature of the cooling water
discharged the fuel cell stack 1 (hereinafter, referred to as a
"stack outlet water temperature"). The stack outlet water
temperature sensor 44 outputs a detection signal indicating the
detected temperature to the controller 110.
[0083] The stack resistance measuring device 45 measures an
internal resistance (HFR: High Frequency Resistance) of the fuel
cell stack 1 to estimate a degree of wetness of the electrolyte
membranes constituting the fuel cells laminated in the fuel cell
stack 1. The smaller the degree of wetness of the electrolyte
membranes, i.e. the less moisture in the electrolyte membranes and
the drier the electrolyte membranes, the larger the internal
resistance. On the other hand, the larger the degree of wetness of
the electrolyte membranes, i.e. the more moisture in the
electrolyte membranes and the wetter the electrolyte membranes, the
smaller the internal resistance.
[0084] For example, the stack resistance measuring device 45
supplies an AC current to a positive electrode terminal of the fuel
cell stack 1 and detects an AC voltage between the positive
electrode terminal and a negative electrode terminal by the AC
current. Then, the stack resistance measuring device 45 calculates
the internal resistance by dividing an amplitude of the AC voltage
by an amplitude of the AC current, and outputs a value of the
internal resistance, i.e. HFR to the controller 110.
[0085] The controller 110 is configured by a microcomputer
including a central processing unit (CPU), a read-only memory
(ROM), a random access memory (RAM) and an input/output interface
(I/O interface).
[0086] To the controller 110 are input detection values output from
the temperature sensor 41, the pressure sensor 42, the stack inlet
water temperature sensor 43, the stack outlet water temperature
sensor 44 and the stack resistance measuring device 45.
[0087] The controller 110 controls the compressor 12, the cathode
pressure control valve 14, the bypass valve 16, the anode pressure
control valve 24 and the purge valve 29 on the basis of input
values, required power required from the electric loads 4 and
command values to the auxiliary machines. In this way, the cathode
gas and the anode gas are supplied to the fuel cell stack 1 and a
power generation state of the fuel cell stack 1 is satisfactorily
maintained.
[0088] The controller 110 executes a control of warming up the fuel
cell stack 1 to a temperature suitable for power generation
(hereinafter, referred to as a "warm-up operation") when the fuel
cell system 100 is started.
[0089] In the warm-up operation, the controller 110 electrically
connects the fuel cell stack 1 to the auxiliary machines and causes
the fuel cell stack 1 to generate power necessary to drive the
auxiliary machines. Since the fuel cell stack 1 generates heat due
to power generation, the fuel cell stack 1 itself is warmed. Power
generated by the fuel cell stack 1 is supplied to the auxiliary
machines such as the compressor 12, the cooling water pump 32 and
the heater 35.
[0090] When such a fuel cell system is started in a sub-zero
temperature environment and the warm-up operation is started, a
temperature difference between the temperature of the cooling water
flowing in the fuel cell stack 1 and that of the fuel cell stack 1
generating heat increases. If a flow rate of the cooling water to
be supplied to the fuel cell stack 1 is increased by increasing a
rotation speed of the cooling water pump 32 in this state, the
amount of heat radiated from the fuel cell stack 1 to the cooling
water increases and the temperature of the fuel cell stack 1 is
less likely to increase. Thus, when the warm-up operation is
started, it is desirable to suppress the rotation speed of the
cooling water pump 32 low.
[0091] On the other hand, if the rotation speed of the cooling
water pump 32 is suppressed low, a temperature increasing rate of
the cooling water is slowed. Thus, the amount of heat radiated from
the cooling water to the anode gas by the heat exchanger 23
decreases and the temperature increasing rate of the anode gas
supplied from the heat exchanger 23 is slowed.
[0092] During sub-zero start, the temperature of the anode gas
supplied from the high-pressure tank 21 to the jet pump 25 could
also reach -30.degree. C. If the anode off-gas is circulated to the
fuel cell stack 1 by the jet pump 25 in such a situation, steam in
the anode off-gas becomes liquid water and this liquid water is
frozen to generate ice in a part where the anode gas and the anode
off-gas join.
[0093] Thus, when the temperature of the anode gas to be supplied
to the jet pump 25 increases at a slower rate, ice formed in the
jet pump 25 increases to close the flow passage in the jet pump 25
and it may not be possible to supply the anode gas to the fuel cell
stack 1.
[0094] Accordingly, in the present embodiment, the controller 110
predicts the freezing of the jet pump 25 and controls a flow rate
of the cooling water to be supplied to the heat exchanger 23 when
the fuel cell system 100 is started below a freezing point.
[0095] FIG. 2 is a block diagram showing a basic configuration of
the controller 110 in the embodiment of the present invention.
[0096] The controller 110 includes a cooling water flow rate
control unit 200 configured to control a flow rate of the cooling
water to be circulated to the fuel cell stack 1 (hereinafter,
referred to as a "cooling water flow rate").
[0097] The cooling water flow rate control unit 200 includes a
normal control unit 210, a stack warm-up control unit 220, a gas
flow passage freezing prevention control unit 230, a switching unit
300 and a cooling water flow rate commanding unit 400.
[0098] The normal control unit 210 controls the cooling water flow
rate on the basis of the cooling water temperature of the fuel cell
stack 1 such that the fuel cell stack 1 is maintained at a
temperature suitable for power generation, e.g. 60.degree. C.
during a normal operation performed after the warm-up operation is
completed. The normal control unit 210 increases the cooling water
flow rate as the temperature of the fuel cell stack 1 increases due
to power generation.
[0099] It should be noted that the normal control unit 210 may
control the cooling water flow rate to maintain the electrolyte
membranes in a wet/dry state determined in advance on the basis of
the HFR of the fuel cell stack 1. For example, the normal control
unit 210 increases the cooling water flow rate as the HFR
increases. In this way, the temperature of the fuel cell stack 1
decreases and a flow rate of steam carried out from the fuel cell
stack 1 by the cathode gas decreases, wherefore the electrolyte
membranes are likely to become wet. In this case, the normal
control unit 210 controls the cooling water flow rate on the basis
of the larger one of a target flow rate based on the cooling water
temperature and a target flow rate based on the HFR.
[0100] The stack warm-up control unit 220 constitutes a warm-up
control unit configured to control the flow rate of the cooling
water to be supplied to the fuel cell stack 1 to a flow rate
determined in advance during the warm-up of the fuel cell stack
1.
[0101] The stack warm-up control unit 220 makes the cooling water
flow rate lower than the flow rate set by the normal control unit
210 when the temperature of the fuel cell stack 1 (hereinafter,
referred to as a "stack temperature") is lower than a warm-up
completion temperature, e.g. 60.degree. C. Since the heat of the
fuel cell stack 1 generating heat is hard to be deprived of by the
cooling water in this way, the warm-up of the fuel cell stack 1 is
promoted.
[0102] A temperature of the cooling water correlated with the
temperature of the fuel cell stack 1, e.g. an average value of the
stack inlet water temperature and the stack outlet water
temperature is used as the stack temperature in the present
embodiment. It should be noted that a temperature sensor may be
directly provided for the fuel cell stack 1 and a detection signal
output from that temperature sensor may be used.
[0103] The gas flow passage freezing prevention control unit 230
controls the cooling water flow rate to prevent the freezing of the
jet pump 25. The gas flow passage freezing prevention control unit
230 increases the flow rate of the cooling water to be supplied to
the heat exchanger 23 when the cooling water flow rate is
controlled by the stack warm-up control unit 220. Since the
temperature increasing rate of the anode gas passing through the
heat exchanger 23 is increased in this way, the anode gas
temperature after joining can be caused to reach the freezing point
early.
[0104] Specifically, the gas flow passage freezing prevention
control unit 230 constitutes a gas temperature increase control
unit configured to increase the temperature of the anode gas
discharged from the jet pump 25.
[0105] The switching unit 300 determines on the basis of the stack
temperature whether or not it is necessary to warm up the fuel cell
stack 1. The switching unit 300 switches the control unit for
controlling the cooling water flow rate from the normal control
unit 210 to the stack warm-up control unit 220 when determining
that the warm-up is necessary.
[0106] Further, the switching unit 300 predicts whether or not the
jet pump 25 will be frozen, on the basis of the temperature of the
anode off-gas before joining to be circulated from the fuel cell
stack 1 to the jet pump 25 (hereinafter, referred to as a
"circulating gas temperature").
[0107] The stack inlet water temperature corrected with the
circulating gas temperature is, for example, used as the
circulating gas temperature. It should be noted that a temperature
sensor configured to detect the temperature of the anode off-gas
may be provided in the anode gas circulation passage 26 and a
detection signal output from that temperature sensor may be
used.
[0108] The switching unit 300 switches the control unit for
controlling the cooling water flow rate from the stack warm-up
control unit 220 to the gas flow passage freezing prevention
control unit 230 when predicting that the jet pump 25 will be
frozen.
[0109] Furthermore, the switching unit 300 determines whether or
not a state where ice will be generated in the jet pump 25 is set,
on the basis of the temperature of the anode gas discharged from
the jet pump 25 to the fuel cell stack 1 (hereinafter, referred to
a "discharged gas temperature"). It should be noted that the
discharged gas temperature is the temperature of post-joining gas
after the circulating gas and the supplied gas are joined.
[0110] The discharged gas temperature is calculated on the basis of
a target current, the circulating gas temperature and the supplied
gas temperature. A calculation method of the discharged gas
temperature is described in detail later with reference to FIG. 4.
It should be noted that a temperature sensor may be provided
between the jet pump 25 and the fuel cell stack 1 in the anode gas
supply passage 22 and a detection signal output from that
temperature sensor may be used.
[0111] The switching unit 300 switches the control unit for
controlling the cooling water flow rate to the gas flow passage
freezing prevention control unit 230 when determining the state
where ice will be generated in the jet pump 25. On the other hand,
the switching unit 300 switches the control unit for controlling
the cooling water flow rate to the stack warm-up control unit 220
when determining a state where ice will not be generated in the jet
pump 25.
[0112] The cooling water flow rate commanding unit 400 calculates a
rotation speed of the cooling water pump 32 on the basis of the
cooling water flow rate set by the normal control unit 210, the
stack warm-up control unit 220 or the gas flow passage freezing
prevention control unit 230 and outputs a command signal commanding
that rotation speed to the cooling water pump 32.
[0113] FIG. 3 is a flow chart showing an example of a control
method of the cooling water flow rate control unit 200 in the
present embodiment.
[0114] In Step S101, the cooling water flow rate control unit 200
detects the stack temperature. Specifically, the cooling water flow
rate control unit 200 calculates an average value of a detection
value of the stack inlet water temperature sensor 43 and that of
the stack outlet water temperature sensor 44 as the stack
temperature.
[0115] In Step S102, the switching unit 300 judges whether or not
the stack temperature is lower than a warm-up determination
threshold value. The warm-up determination threshold value is set
at the temperature suitable for the power generation of the fuel
cell stack 1, e.g. 60.degree. C.
[0116] In Step S103, the switching unit 300 sets a stack warm-up
flag on if the stack temperature is lower than the warm-up
determination threshold value.
[0117] In Step S104, the switching unit 300 controls the flow rate
of the cooling water to be supplied to the fuel cell stack 1 to a
warm-up request flow rate determined in advance if the stack
warm-up flag is set on. Since a temperature difference between the
fuel cell stack 1 generating heat and the cooling water increases
if the fuel cell system 100 is started when the stack temperature
is lower than 0.degree. C., the warm-up request flow rate is set at
a value smaller than the cooling water flow rate during a normal
operation.
[0118] In Step S105, the switching unit 300 judges whether or not
the circulating gas temperature is not lower than a moisture
increase threshold value Th_s. The moisture increase threshold
value Th_s is set on the basis of a temperature at which the amount
of steam in the anode off-gas increases, e.g. set at 20.degree.
C.
[0119] If the circulating gas temperature is higher than the
moisture increase threshold value Th_s, the switching unit 300
predicts that ice formed in the jet pump 25 will increase to close
the flow passage (freezing).
[0120] In Step S106, the switching unit 300 judges whether or not
the discharged gas temperature is not higher than a freezing
release threshold value Th_e if the circulating gas temperature is
not lower than the moisture increase threshold value Th_s. The
freezing release threshold value Th_e is set at a value at which
ice is generated in the jet pump 25, e.g. set at 0.degree. C.
[0121] In Step S107, the switching unit 300 sets a gas flow passage
freezing prevention flag on due to a possibility that the jet pump
25 will be frozen if the circulating gas temperature is not lower
than the moisture increase threshold value Th_s and the discharged
gas temperature is not higher than the freezing release threshold
value Th_e.
[0122] In Step S108, the switching unit 300 switches the flow rate
of the cooling water supplied from the cooling water pump 32 to the
heat exchanger 23 to a gas temperature increase request flow rate
if the gas flow passage freezing prevention flag is set on.
[0123] The gas temperature increase request flow rate is a flow
rate determined to remove ice generated in the jet pump 25, and set
at a value larger than the warm-up request flow rate. By setting
the cooling water flow rate to the gas temperature increase request
flow rate, the flow rate of the cooling water to be supplied to the
heat exchanger 23 increases and the anode gas to be supplied to the
jet pump 25 is heated in the heat exchanger 23, wherefore the
discharged gas temperature increases at a faster rate.
[0124] Subsequently, the switching unit 300 returns to Step S106
and sets the cooling water flow rate higher than the warm-up
request flow rate until the discharged gas temperature reaches the
freezing release threshold value Th_e. A return is made to Step
S102 when the discharged gas temperature exceeds the freezing
release threshold value Th_e, and the switching unit 300 sets the
cooling water flow rate back to the warm-up request flow rate
unless the stack temperature is higher than the warm-up
determination threshold value.
[0125] In Step S109, the switching unit 300 sets the stack warm-up
flag off if the stack temperature is judged to be not lower than
the warm-up determination threshold value in Step S102.
[0126] In Step S110, the switching unit 300 sets the stack warm-up
flag off and sets the gas flow passage freezing prevention flag
off.
[0127] In Step S111, the switching unit 300 switches to the normal
control for controlling the cooling water flow rate on the basis of
the electric loads 4 after the fuel cell stack 1 is warmed-up.
[0128] FIG. 4 is a diagram showing a configuration example of a
discharged gas temperature calculation unit 120 configured to
calculate the discharged gas temperature in the controller 110.
[0129] The discharged gas temperature calculation unit 120 includes
a supplied gas flow rate calculation unit 121, a circulating gas
flow rate calculation unit 122, a circulating gas volume ratio
calculation unit 123, a pre-joining supplied gas enthalpy
calculation unit 124, a circulating gas enthalpy calculation unit
125 and a post-joining gas temperature calculation unit 126.
[0130] The supplied gas flow rate calculation unit 121 calculates a
flow rate of the anode gas to be supplied to the fuel cell stack 1
(hereinafter, referred to as a "supplied gas flow rate") on the
basis of the target current of the fuel cell stack 1. For example,
the supplied gas flow rate calculation unit 121 calculates the
supplied gas flow rate from a map determined in advance when
receiving the target current.
[0131] The target current of the fuel cell stack 1 is calculated on
the basis of power required from the electric loads 4 such as an
electric motor or an auxiliary machine. For example, as a depressed
amount of an accelerator pedal increases, power required from the
electric motor increases, wherefore the target current
increases.
[0132] The circulating gas flow rate calculation unit 122
calculates a circulating gas flow rate with reference to a map
determined in advance on the basis of the target current of the
fuel cell stack 1 and a purge flow rate. The purge flow rate is
calculated on the basis of an opening degree of the purge valve 29
and the like.
[0133] The circulating gas volume ratio calculation unit 123
calculates a volume ratio of hydrogen gas, nitrogen gas and stream
in the circulating gas.
[0134] Specifically, the circulating gas volume ratio calculation
unit 123 calculates a stack outlet gas pressure by subtracting a
pressure loss of the fuel cell stack 1 from a stack inlet gas
pressure, and calculates a steam volume ratio by dividing the stack
outlet gas pressure by a saturated stream pressure obtained from
the circulating gas temperature. The stack temperature correlated
with the circulating gas temperature is used as the circulating gas
temperature in the present embodiment.
[0135] Further, the circulating gas volume ratio calculation unit
123 calculates a hydrogen gas volume ratio in the circulating gas
from a map determined in advance on the basis of the target
current. Then, the circulating gas volume ratio calculation unit
123 calculates a nitrogen gas volume ratio from the hydrogen gas
volume ratio and the steam volume ratio in the circulating gas.
[0136] The pre-joining supply gas enthalpy calculation unit 124
calculates an enthalpy of the pre joining supplied gas from a
predetermined mathematical formula or the like on the basis of a
pre-joining supplied gas flow rate and the supplied gas
temperature. A pre joining hydrogen flow rate is a value obtained
by subtracting a hydrogen gas flow rate in the circulating gas from
the supplied gas flow rate. The supplied gas temperature is the
temperature of the anode gas to be supplied to the jet pump 25 and
calculated on the basis of a detection signal output from the
temperature sensor 41.
[0137] The circulating gas enthalpy calculation unit 125 calculates
an enthalpy of the circulating gas from a predetermined
mathematical formula or the like on the basis of the flow rate of
each of the hydrogen gas, the nitrogen gas and the stream gas in
the circulating gas and the circulating gas temperature.
[0138] The post-joining gas temperature calculation unit 126
calculates the temperature of the post-joining gas obtained by
joining the pre joining supplied gas and the pre joining
circulating gas in the jet pump 25.
[0139] Specifically, the post-joining gas temperature calculation
unit 126 calculates a total enthalpy of the pre joining gas by
adding the enthalpy of each of the pre joining supplied gas and the
circulating gas. The post joining gas temperature calculation unit
126 integrates a heat capacity obtained by multiplying specific
heat of the hydrogen gas by the pre joining supplied gas flow rate,
a heat capacity obtained by multiplying a nitrogen gas flow rate in
the circulating gas by specific heat of the nitrogen gas and a heat
capacity obtained by multiplying a steam flow rate in the
circulating gas by steam specific heat on the basis of the volume
ratio of the circulating gas. The post-joining gas temperature
calculation unit 126 calculates the gas temperature after joining
by dividing the total enthalpy before joining by the integrated
heat capacity.
[0140] According to the first embodiment of the present invention,
the fuel cell system 100 includes the cooling water pump 32
configured to supply the cooling water (refrigerant) to the fuel
cell stack 1 and the heat exchanger 23 configured to exchange heat
between the cooling water increased in temperature in the fuel cell
stack 1 and the anode gas flowing in the anode gas supply passage
22. Further, the fuel cell system 100 includes the jet pump 25 as a
component for circulating the anode off-gas discharged from the
fuel cell stack 1 to the fuel cell stack 1.
[0141] In such a fuel cell system, the stack warm-up control unit
220 controls the flow rate of the cooling water to be supplied to
the fuel cell system 1 to the warm-up request flow rate determined
in advance when the fuel cell stack 1 is warmed up, i.e. when the
stack temperature is lower than the warm-up determination threshold
value in the present embodiment. The warm-up request flow rate is
set at the value smaller than the flow rate set by the normal
control unit 210.
[0142] The gas flow passage freezing prevention control unit 230
increases the flow rate of the cooling water to be supplied to the
heat exchanger 23 from the warm-up request flow rate on the basis
of the cooling water temperature of the fuel cell stack 1
correlated with the temperature of the anode off-gas when the
cooling water flow rate is controlled by the stack warm-up control
unit 220.
[0143] Since the temperature increasing rate of the anode gas
heated by the heat exchanger 23 is increased during the warm-up of
the fuel cell stack 1 in this way, it is possible to reduce the
amount of ice generated when the anode gas to be supplied from the
heat exchanger 23 and the anode off-gas join.
[0144] Thus, it can be avoided that the gas flow passage is closed
by ice generated during the warm-up of the fuel cell stack 1.
Therefore, it is possible to prevent the freezing of the component
for circulating the gas discharged from the fuel cell stack 1 while
realizing early warm-up of the fuel cell stack 1.
[0145] It should be noted that although an example of increasing
the flow rate of the cooling water to be supplied to the heat
exchanger 23 from the warm-up request flow rate on the basis of the
cooling water temperature of the fuel cell stack 1 has been
described in the present embodiment, there is no limitation to
this. For example, a temperature sensor may be provided in the
anode gas circulation passage 26 and the cooling water to be
supplied to the heat exchanger 23 may be increased from the warm-up
request flow rate on the basis of a detection signal output from
this temperature sensor.
[0146] In the present embodiment, the control unit configured to
control the cooling water flow rate when the temperature of the
anode off-gas (circulating gas temperature) exceeds the moisture
increase threshold value Th_s is switched from the stack warm-up
control unit 220 to the gas flow passage freezing prevention
control unit 230. Then, the gas flow passage freezing prevention
control unit 230 makes the flow rate of the cooling water to be
supplied to the heat exchanger 23 higher than the warm-up request
flow rate. The moisture increase threshold value Th_s is set at a
temperature at which the amount of steam in the anode off-gas
largely increases, i.e. at a temperature equal to or higher than
0.degree. C.
[0147] Since the cooling water flow rate is not increased until the
amount of stream in the anode off-gas increases in this way, an
increase of power consumption of the cooling water pump 32 can be
avoided while the warm-up of the fuel cell stack 1 is promoted.
[0148] It should be noted that although an example of using the jet
pump 25 as a component for circulating the anode gas has been
described in the present embodiment, a compressor, a pump or the
like may be used. Further, although the fuel cell system 100 of the
present embodiment is configured to circulate the anode off-gas to
the fuel cell stack 1, effects and functions similar to those of
the present embodiment can be obtained even if the cathode off-gas
is circulated to the fuel cell stack 1.
[0149] As described above, the freezing of the gas flow passage can
be prevented by increasing the cooling water flow rate on the basis
of the temperature of the discharged gas during the warm-up in the
fuel cell system for circulating at least one discharged gas, out
of the anode off-gas and the cathode off-gas, to the fuel cell
stack 1.
Second Embodiment
[0150] FIG. 5 is a block diagram showing a detailed configuration
of a cooling water flow rate control unit 200 in a second
embodiment of the present invention.
[0151] A fuel cell system of the present embodiment basically has
the same configuration as the fuel cell system 100 shown in FIG. 1.
In the following description, the same components as in the fuel
cell system 100 are denoted by the same reference signs and not
described.
[0152] The cooling water flow rate control unit 200 includes a
normal control flow rate calculation unit 211, a warm-up request
flow rate calculation unit 221, a subtractor 231, a gas temperature
increase request flow rate calculation unit 232, a flow rate
correction value calculation unit 233, a multiplier 234, a cooling
water temperature difference calculation unit 241 and a stack
excessive temperature increase preventing flow rate calculation
unit 242. Further, the cooling water flow rate control unit 200
includes a switcher 310, a switcher 320, a release value holding
unit 321, a request flow rate setting unit 330 and a cooling water
target flow rate setting unit 340.
[0153] The normal control unit 211 calculates a cooling water flow
rate for properly maintaining the temperature of a fuel cell stack
1 after the warm-up of the fuel cell stack 1 is completed
(hereinafter, referred to as a "normal control flow rate"). The
normal control flow rate calculation unit 211 increases a normal
operation flow rate as a target current of the fuel cell stack 1
increases. It should be noted that the normal control flow rate
calculation unit 211 constitutes a normal control unit 210
configured to control the cooling water flow rate on the basis of
electric loads 4.
[0154] In the present embodiment, a normal operation map indicating
a relationship between the target current of the fuel cell stack 1
and the normal operation flow rate is stored in advance in the
normal control flow rate calculation unit 211. The normal control
flow rate calculation unit 211 refers to this normal operation map
and calculates the normal operation flow rate associated with the
target current when obtaining the target current.
[0155] The warm-up request flow rate calculation unit 221
calculates a cooling water flow rate for warming up the fuel cell
stack 1 (hereinafter, referred to as a "warm-up request flow
rate"). The warm-up request flow rate is set at a value smaller
than the normal control flow rate. Further, the warm-up request
flow rate calculation unit 221 reduces the warm-up request flow
rate as the temperature of the fuel cell stack 1 decreases.
[0156] In this way, the amount of heat radiated from the fuel cell
stack 1 generating heat by power generation to the cooling water is
suppressed as the temperature of the fuel cell stack 1 decreases.
Thus, the warm-up of the fuel cell stack 1 can be promoted. It
should be noted that the warm-up request flow rate calculation unit
221 constitutes a stack warm-up control unit 220 configured to make
the cooling water flow rate smaller than the normal control flow
rate when the fuel cell stack 1 is warmed up.
[0157] In the present embodiment, a warm-up operation map
indicating a relationship between the cooling water temperature
correlated with the temperature of the fuel cell stack 1 and the
warm-up request flow rate is stored in advance in the warm-up
request flow rate calculation unit 221. The warm-up request flow
rate calculation unit 221 refers to the warm-up operation map and
calculates the warm-up request flow rate associated with the
cooling water temperature when obtaining the cooling water
temperature.
[0158] The switcher 310 switches a value to be output to the
request flow rate setting unit 330 to the normal control flow rate
or the warm-up request flow rate according to a set state of a
stack warm-up flag.
[0159] The switcher 310 outputs the warm-up request flow rate to
the request flow rate setting unit 330 if the stack warm-up flag is
set on in Step S103 shown in FIG. 3. On the other hand, the
switcher 310 outputs the normal control flow rate to the request
flow rate setting unit 330 if the stack warm-up flag is set
off.
[0160] The subtractor 231 calculates a temperature difference
.DELTA.T by subtracting a supplied gas temperature from a stack
inlet water temperature. The supplied gas temperature is a
parameter correlated with a discharged gas temperature during
sub-zero start and detected by a temperature sensor 41 shown in
FIG. 1. It should be noted that the discharged gas temperature
calculated by the discharged gas temperature calculation unit 120
shown in FIG. 4 may be used instead of the supplied gas
temperature.
[0161] The stack inlet water temperature is a parameter correlated
with the temperature of the anode off-gas (circulating gas
temperature) and detected by the stack inlet water temperature
sensor 43 shown in FIG. 1.
[0162] By using a detection value of the stack inlet water
temperature sensor 43 instead of the temperature of the anode
off-gas, it is not necessary to newly provide a temperature sensor
in an anode gas circulation passage 26 and prepare a measure to
prevent the freezing of that temperature sensor. Thus, an increase
of manufacturing cost can be suppressed.
[0163] It should be noted that the fuel cell stack 1 is assumed to
be a counter-flow type fuel cell stack formed such that an anode
gas outlet hole and a cooling water inlet hole are adjacent in the
present embodiment. In contrast, in the case of using a fuel cell
stack formed such that an anode gas outlet hole and a cooling water
outlet hole are adjacent, it is desirable to use a stack outlet
water temperature instead of the stack inlet water temperature.
Further, a temperature sensor may be provided in the anode gas
circulation passage 26 and a detection signal output from that
temperature sensor may be used.
[0164] The gas temperature increase request flow rate calculation
unit 232 calculates a cooling water flow rate for more quickly
increasing the temperature of the anode gas warmed by the heat
exchanger 23 than during the warm-up operation (hereinafter,
referred to as a "gas temperature increase request flow rate"). The
gas temperature increase request flow rate is set at a value larger
than the warm-up request flow rate.
[0165] The gas temperature increase request flow rate calculation
unit 232 increases the gas temperature increase request flow rate
since an effect of increasing the temperature of the anode gas
increases by increasing the cooling water flow rate as the
temperature difference .DELTA.T between the stack inlet water
temperature and the supplied gas temperature increases.
[0166] Further, the gas temperature increase request flow rate
calculation unit 232 increases the gas temperature increase request
flow rate as the cooling water temperature when the fuel cell
system 100 is started decreases since a longer time is required to
increase the discharged gas temperature to the freezing point. By
increasing the gas temperature increase request flow rate, a
temperature increasing time is shortened. Thus, the closure of the
flow passage by ice generated in a jet pump 25 can be
suppressed.
[0167] It should be noted that the gas temperature increase request
flow rate calculation unit 232 constitutes a gas flow passage
freezing prevention control unit 230 configured to make the cooling
water flow rate higher than the warm-up request flow rate on the
basis of the temperature of the anode off-gas.
[0168] In the present embodiment, a freezing prevention control map
indicating a relationship between the temperature difference
.DELTA.T and the gas temperature increase request flow rate is
stored in advance in the gas temperature increase request flow rate
calculation unit 232. The freezing prevention control map is
described later with reference to FIG. 6.
[0169] The gas temperature increase request flow rate calculation
unit 232 refers to the freezing prevention control map and
calculates the gas temperature increase request flow rate
associated with the temperature difference .DELTA.T at the cooling
water temperature during the start when obtaining the cooling water
temperature and the temperature difference .DELTA.T during the
start. The gas temperature increase request flow rate calculation
unit 232 outputs that gas temperature increase request flow rate to
the multiplier 234.
[0170] The flow rate correction value calculation unit 233
calculates a correction value for correcting the gas temperature
increase request flow rate. The flow rate correction value
calculation unit 233 calculates the correction value on the basis
of the target current and HFR of the fuel cell stack 1.
[0171] For example, the flow rate correction value calculation unit
233 increases the correction value to increase the gas temperature
increase request flow rate as the target current increases since
the cooling water temperature increases and an effect of increasing
the temperature of the anode gas increases.
[0172] Further, the flow rate correction value calculation unit 233
increases the correction value to increase the gas temperature
increase request flow rate since the amount of steam contained in
the anode off-gas increases as the HFR decreases.
[0173] In the present embodiment, a correction map indicating a
relationship between the target current and the gas temperature
increase request flow rate for each HFR is stored in advance in the
flow rate correction value calculation unit 233. The correction map
is described with reference to FIG. 7 later.
[0174] When obtaining the target current and the HFR, the flow rate
correction value calculation unit 233 refers to the correction map
specified by that HFR and calculates a correction value associated
with that target current. The flow rate correction value
calculation unit 233 outputs that correction value to the
multiplier 234.
[0175] The multiplier 234 corrects the gas temperature increase
request flow rate by multiplying the gas temperature increase
request flow rate by the correction value. The multiplier 234
outputs the corrected gas temperature increase request flow rate to
the switcher 320.
[0176] The release value holding unit 321 holds zero as a value for
releasing a freezing prevention control.
[0177] The switcher 320 switches a value to be output to the
request flow rate setting unit 330 to the corrected gas temperature
increase request flow rate or zero according to a set state of a
gas flow passage freezing prevention flag.
[0178] The switcher 320 outputs the gas temperature increase
request flow rate to the request flow rate setting unit 330 if the
gas flow passage freezing prevention flag is set on in Step S107
shown in FIG. 3. On the other hand, the switcher 320 outputs zero
to the request flow rate setting unit 330 to release the freezing
prevention control if the gas flow passage freezing prevention flag
is set off.
[0179] The request flow rate setting unit 330 sets the larger one
of the normal control flow rate or the warm-up request flow rate
output from the switcher 310 and the gas temperature increase
request flow rate or zero output from the switcher 320 as the
request flow rate and outputs that request flow rate to the cooling
water target flow rate setting unit 340.
[0180] For example, the request flow rate setting unit 330 outputs
the warm-up request flow rate as the request flow rate of the
cooling water if the stack warm-up flag is set on with the gas flow
passage freezing prevention flag set off. When the gas flow passage
freezing prevention flag is switched on with the stack warm-up flag
set on, the request flow rate setting unit 330 outputs the gas
temperature increase request flow rate larger than the warm-up
request flow rate.
[0181] Since the flow rate of the cooling water to be supplied to
the heat exchanger 23 increases in this way, the amount of heat
radiated to the anode gas passing through the heat exchanger 23
increases and a temperature increasing time until the discharged
gas temperature of the anode gas discharged from the jet pump 25
reaches a freezing point can be shortened. Thus, the closure of the
flow passage by ice formed in the jet pump 25 can be avoided.
[0182] Further, since the gas flow passage freezing prevention flag
is set off when the discharged gas temperature exceeds a freezing
release threshold value Th_e, the request flow rate setting unit
330 switches the request flow rate of the cooling water from the
gas temperature increase request flow rate to the warm-up request
flow rate.
[0183] In this way, the power consumption of the cooling water pump
32 can be prevented from being unnecessarily increased by
increasing the cooling water flow rate in a state where the jet
pump 25 will not be frozen.
[0184] The cooling water temperature difference calculation unit
241 calculates the cooling water temperature difference between the
inlet and outlet of the fuel cell stack 1 by subtracting the stack
inlet water temperature from the stack outlet water temperature and
outputs that cooling water temperature difference to the stack
excessive temperature increase preventing flow rate calculation
unit 242.
[0185] The stack excessive temperature increase preventing flow
rate calculation unit 242 calculates a cooling water flow rate for
preventing the temperature of the fuel cell stack 1 from becoming
excessively high when the fuel cell system 100 is started
(hereinafter, referred to as an "excessive temperature increase
preventing flow rate"). The excessive temperature increase
preventing flow rate during a start-up processing is set at a value
smaller than the normal control flow rate.
[0186] The stack excessive temperature increase preventing flow
rate calculation unit 242 increases the excessive temperature
increase freezing prevention as the target current of the fuel cell
stack 1 increases since the amount of generated heat associated
with the power generation of the fuel cell stack 1 increases.
Further, the stack excessive temperature increase preventing flow
rate calculation unit 242 increases the excessive temperature
increase preventing flow rate to lower the temperature on the
outlet side of the fuel cell stack 1 to the temperature on the
inlet side as the cooling water temperature difference
increases.
[0187] In the present embodiment, an excessive temperature increase
prevention map indicating a relationship between the target current
and the excessive temperature increase preventing flow rate for
each cooling water temperature difference is stored in advance in
the stack excessive temperature increase preventing flow rate
calculation unit 242. The excessive temperature increase prevention
map is described later with reference to FIG. 8.
[0188] The stack excessive temperature increase preventing flow
rate calculation unit 242 refers to the excessive temperature
increase prevention map specified by the cooling water temperature
difference and calculates the excessive temperature increase
preventing flow rate associated with the target current when
obtaining the cooling water temperature difference and the target
current. The stack excessive temperature increase preventing flow
rate calculation unit 242 outputs that excessive temperature
increase preventing flow rate to the cooling water target flow rate
setting unit 340.
[0189] The cooling water target flow rate setting unit 340 sets the
larger one of the excessive temperature increase preventing flow
rate and the value output from the request flow rate setting unit
330 as the cooling water target flow rate.
[0190] For example, when the excessive temperature increase
preventing flow rate becomes larger than the warm-up request flow
rate in the case where the stack warm-up flag is set on, the
cooling water target flow rate setting unit 340 sets the excessive
temperature increase freezing prevention as the cooling water
target flow rate. In this way, the temperature of the fuel cell
stack 1 can be prevented from becoming excessively high by the
warm-up request flow rate.
[0191] FIG. 6 is a graph showing an example of the freezing
prevention control map stored in the gas temperature increase
request flow rate calculation unit 232.
[0192] As shown in FIG. 6, the temperature difference .DELTA.T
calculated by the subtractor 231 and the gas temperature increase
request flow rate are associated with each other for each cooling
water temperature during the start in the freezing prevention
control map. The temperature difference .DELTA.T is a difference
between the temperature of the gas supplied from the heat exchanger
23 to the jet pump 25 and that of the gas circulated from the fuel
cell stack 1 to the jet pump 25.
[0193] In the freezing prevention control map, the cooling water
flow rate increases as the temperature difference .DELTA.T
increases. This is because the amount of heat radiated from the
cooling water to the anode gas increases by increasing the cooling
water flow rate as the temperature difference between the anode gas
and the cooling water increases in the heat exchanger 23.
[0194] Further, at the same temperature difference .DELTA.T, the
gas temperature increase request flow rate increases as the cooling
water temperature during the start decreases. This is to suppress a
situation where an increase width of the anode gas temperature to
the freezing release threshold value Th_e increases and the
temperature increasing time becomes longer as the cooling water
temperature during the system start decreases.
[0195] FIG. 7 is a graph showing an example of the correction map
stored in the flow rate correction value calculation unit 233.
[0196] As shown in FIG. 7, the target current and the correction
value are associated with each other for each HFR of the fuel cell
stack 1 in the correction map.
[0197] In the correction map, as the target current increases, the
correction value increases to increase the gas temperature increase
request flow rate. This is because the temperature of the anode gas
is likely to increase by increasing the flow rate of the cooling
water to be supplied to the heat exchanger 23 as the target current
increases since the cooling water temperature increases due to the
heat generation of the fuel cell stack 1 and the temperature
difference .DELTA.T becomes larger.
[0198] Further, at the same target current, the correction value
decreases to decrease the gas temperature increase request flow
rate as the HFR increases. This is because an increasing amount of
ice generated in the jet pump 25 decreases since the amount of
steam in the anode off-gas to be sucked into the jet pump 25
decreases as the HFR increases, i.e. as the fuel cells become
drier.
[0199] FIG. 8 is a graph showing an example of the excessive
temperature increase prevention map stored in the stack excessive
temperature increase preventing flow rate calculation unit 242.
[0200] As shown in FIG. 8, the target current of the fuel cell
stack 1 and the excessive temperature increase preventing flow rate
are associated with each other for each cooling water temperature
difference in the excessive temperature increase prevention.
[0201] In the excessive temperature increase prevention map, the
excessive temperature increase preventing flow rate increases as
the target current increases. This is to suppress a sudden
temperature increase of the fuel cell stack 1 since the amount of
heat generation of the fuel cell stack 1 increases as the target
current increases.
[0202] Further, at the same target current, the gas temperature
increase request flow rate increases as the cooling water
temperature difference increases. This is to reduce the temperature
on the outlet side of the fuel cell stack 1 as the cooling water
temperature difference increases since the fuel cells cannot be
cooled on the outlet side of the fuel cell stack 1 as much as on
the inlet side.
[0203] Next, the operation of the cooling water flow rate control
unit 200 in the present embodiment is described with reference to
FIGS. 9 and 10.
[0204] FIG. 9 are time charts when the freezing prevention control
of the jet pump 25 is executed by the cooling water flow rate
control unit 200.
[0205] FIG. 9(a) is a chart showing a change of an operating state
of the fuel cell system 100. FIG. 9(b) is a chart showing a change
of each of the temperature of the anode off-gas before joining to
be sucked into the jet pump 25 (circulating gas temperature), the
temperature of the anode gas before joining to be supplied to the
jet pump 25 (supplied gas temperature) and the temperature of the
anode gas after the supplied gas and the circulating gas before
joining are joined in the jet pump 25 (post-joining gas
temperature).
[0206] Here, a change of the cooling water temperature is shown as
the circulating gas temperature before joining. It should be noted
that the cooling water temperature is the temperature of the
cooling water detected by the stack inlet water temperature sensor
43 and the supplied gas temperature is the temperature of the anode
gas detected by the temperature sensor 41. The post joining gas
temperature is the temperature of the anode gas after joining
discharged from the jet pump 25.
[0207] FIG. 9(c) is a chart showing a change of the flow rate of
the cooling water discharged from the cooling water pump 32. In
FIG. 9(c), the warm-up request flow rate is shown by a broken line
and the gas temperature increase request flow rate is shown by a
dashed-dotted line. FIG. 9(d) is a chart showing a change of the
amount of ice formed in the jet pump 25.
[0208] A horizontal axis of each of FIGS. 9(a) to 9(d) is a time
axis common to each other. Further, in FIGS. 9(b) and 9(d), changes
when only the warm-up operation is performed without executing the
gas flow passage freezing prevention control are shown by broken
lines.
[0209] Before time t0, the fuel cell system 100 is in a stopped
state as shown in FIG. 9(a), the cooling water temperature is a
temperature much lower than 0.degree. C., e.g. -20.degree. C. and
the supplied gas temperature is a temperature even lower than the
cooling water temperature, e.g. -30.degree. C.
[0210] At time t0, the fuel cell system 100 is started and the
stack warm-up flag is set on and the warm-up operation is performed
since the cooling water temperature is lower than the warm-up
determination threshold value.
[0211] In the warm-up operation, the controller 110 supplies
generated power from the fuel cell stack 1 to the auxiliary
machines such as the compressor 12, the cooling water pump 32 and
the heater 35 and warms up the fuel cell stack 1 by self-heat
generation associated with the power generation of the fuel cell
stack 1 and heat radiation by the heater 35. In this way, the
cooling water temperature gradually increases as shown in FIG.
9(b).
[0212] At this time, as shown in FIG. 9(c), the cooling water flow
rate control unit 200 sets the flow rate of the cooling water
discharged from the cooling water pump 32 as the warm-up request
flow rate. Since the cooling water temperature increases according
to the passage of time, the cooling water flow rate control unit
200 monotonously increases the warm-up request flow rate with the
passage of time. It should be noted that the cooling water flow
rate control unit 200 may increase the warm-up request flow rate
according to a change of the cooling water temperature.
[0213] At time t1, the cooling water temperature exceeds the
moisture increase threshold value Th_s as shown in FIG. 9(b). Thus,
the gas flow passage freezing prevention flag is set on. Associated
with this, the cooling water flow rate control unit 200 increases
the cooling water flow rate from the warm-up request flow rate to
the gas temperature increase request flow rate as shown in FIG.
9(c).
[0214] Since the flow rate of the cooling water to be supplied to
the heat exchanger 23 increases as shown in FIG. 9(b), the anode
gas temperature before joining increases at a faster rate as
compared to the case where only the warm-up operation is performed,
whereby the anode gas temperature after joining also increases at a
faster rate.
[0215] By setting the warm-up request flow rate until a timing at
which ice in the jet pump 25 increases as shown in FIG. 9(d), the
warm-up time of the fuel cell stack 1 can be ensured to be long and
an unnecessary increase of the cooling water flow rate can be
avoided.
[0216] Since the temperature difference .DELTA.T between the
circulating gas temperature and the discharged gas temperature,
i.e. a temperature difference between the circulating gas
temperature and the supplied gas temperature becomes smaller as
time passes from time t1, a temperature increasing effect of the
anode gas by an increase of the cooling water flow rate becomes
smaller. Thus, the cooling water flow rate control unit 200 reduces
the gas temperature increase request flow rate as shown in FIG.
9(c).
[0217] In this way, it can be prevented that the cooling water flow
rate is increased more than necessary in a situation where such an
increase does not contribute to increases of the supplied gas
temperature and the discharged gas temperature. Therefore, the
power consumption of the cooling water pump 32 can be reduced and
the warm-up of the fuel cell stack 1 can be promoted by lowering
the cooling water flow rate.
[0218] After time t2, the anode gas temperature after joining
exceeds the freezing release threshold value Th_e as shown in FIG.
9(b). Thus, the gas flow passage freezing prevention flag is
switched off. Associated with this, the cooling water flow rate
control unit 200 switches the cooling water flow rate from the gas
temperature increase request flow rate to the warm-up request flow
rate as shown in FIG. 9(c). In this way, the warm-up of the fuel
cell stack 1 is promoted.
[0219] Associated with this, the anode gas temperature after
joining temporarily decreases as shown in FIG. 9(b), but increases
again without decreasing to 0.degree. C. (freezing point
temperature). By setting the freezing release threshold value Th_e
higher than 0.degree. C. in this way, the jet pump 25 can be
prevented from being frozen after a switch to the warm-up request
flow rate.
[0220] Further, since the anode gas temperature after joining is
maintained to be higher than 0.degree. C., ice formed in the jet
pump 25 immediately melts as shown in FIG. 9(d).
[0221] By executing the gas flow passage freezing prevention
control at the timing at which the amount of steam in the
circulating gas starts increasing in this way, an execution time of
the gas flow passage freezing prevention control (t1-t2) can be
shortened and a loner time can be ensured for the warm-up
operation. Therefore, it is possible to suppress an increase of the
power consumption of the cooling water pump 32 and promote the
warm-up of the fuel cell stack 1.
[0222] It should be noted that an example of reducing the
temperature difference .DELTA.T between the circulating gas
temperature and the discharged gas temperature is shown in FIG. 9.
An example of increasing the temperature difference .DELTA.T is
briefly described with reference to FIG. 10.
[0223] FIG. 10 are time charts when the temperature difference
.DELTA.T becomes larger while the gas freezing prevention control
is executed.
[0224] A vertical axis of each of FIGS. 10(a) to 10(d) is the same
as that of each of FIGS. 9(a) to 9(d), and a horizontal axis is a
time axis common to each other. Here, a state of the fuel cell
system 100 from time t11 to time t12 is described.
[0225] At time t11, the cooling water temperature exceeds the
moisture increase threshold value Th_s. Thus, the cooling water
flow rate control unit 200 switches the cooling water flow rate to
the gas temperature increase request flow rate as shown in FIG.
10(c). Thereafter, the temperature difference .DELTA.T between the
circulating gas temperature and the discharged gas temperature
becomes larger as shown in FIG. 10(b). Similarly, the temperature
difference between the circulating gas temperature and the supplied
gas temperature also becomes larger.
[0226] In such a case, the anode gas temperature increasing effect
after joining by an increase of the flow rate of the cooling water
to be supplied to the heat exchanger 23 becomes larger. Thus, the
cooling water flow rate control unit 200 increases the gas
temperature increase request flow rate as shown in FIG. 10(c). This
causes the discharged gas temperature to reach the freezing release
threshold value Th_e earlier than the discharged gas temperature
from time t1 to time t2 shown in FIG. 9(b) as shown in FIG.
10(b).
[0227] By increasing the gas temperature increase request flow rate
as the temperature difference .DELTA.T increases in this way, the
discharged gas temperature can be effectively increased to be
higher than the freezing point in a short time.
[0228] By increasing and decreasing the gas temperature increase
request flow rate according to the temperature difference .DELTA.T
as shown in FIGS. 9 and 10, it is possible to prevent the freezing
and closure of the gas flow passage in the jet pump 25 while
suppressing an increase of the power consumption of the cooling
water pump 32.
[0229] According to the second embodiment of the present invention,
the gas temperature increase request flow rate calculation unit 232
increases the cooling water flow rate to be higher than the warm-up
request flow rate calculated by the warm-up request flow rate
calculation unit 221 if the temperature of the anode off-gas to be
sucked into the jet pump 25 exceeds the moisture increase threshold
value Th_s.
[0230] Since the temperature of the anode gas to be supplied from
the heat exchanger 23 to the jet pump 25 increases in this way, ice
formed in the jet pump 25 can be melted before the jet pump 25 is
frozen to close the flow passage.
[0231] Further, in the present embodiment, the gas temperature
increase request flow rate calculation unit 232 increases an
increase width of the cooling water flow rate from the warm-up
request flow rate as the temperature difference between the
temperature of the discharged gas discharged from the jet pump 25
and the temperature of the anode off-gas (circulating gas
temperature) increases.
[0232] Since an increase width of the cooling water flow rate is
increased when the effect of increasing the temperature of the
anode gas in the heat exchanger 23 by an increase of the cooling
water flow rate is large, the cooling water pump 32 can be
efficiently driven.
[0233] Further, in the present embodiment, the flow rate correction
value calculation unit 233 increases the correction value to
increase the gas temperature increase request flow rate as the
target current correlated with the supply flow rate of the anode
gas increases as shown in FIG. 7. Specifically, the flow rate
correction value calculation unit 233 increases an increase width
from the warm-up flow rate to the gas temperature increase request
flow rate as the flow rate of the gas to be supplied to the fuel
cell stack 1 increases.
[0234] To ensure a temperature increasing rate of the anode gas
passing through the heat exchanger 23, the amount of heat radiated
from the cooling water to the anode gas in the heat exchanger 23
needs to be increased as the flow rate of the anode gas increases.
Thus, the temperature of the anode gas can be quickly and reliably
increased by correcting the gas temperature increase request flow
rate to increase an increase width from the warm-up request flow
rate as the flow rate of the anode gas increases.
[0235] Further, in the present embodiment, the flow rate correction
value calculation unit 233 increases the correction value to
increase the gas temperature increase request flow rate as the HFR
correlated with the degree of wetness of the electrolyte membranes
of the fuel cells increases as shown in FIG. 7. Specifically, the
flow rate correction value calculation unit 233 reduces an increase
width of the cooling water flow rate as the electrolyte membranes
of the fuel cells become drier.
[0236] If the electrolyte membranes are dry, the amount of steam in
the anode off-gas is reduced and an increasing amount of ice formed
in the jet pump 25 is reduced. Thus, by correcting the gas
temperature increase request flow rate to reduce the increase width
of the cooling water flow rate as the electrolyte membranes become
drier, it is possible to reduce the power consumption of the
cooling water pump 32 while suppressing the freezing of the jet
pump 25.
[0237] Further, in the present embodiment, the gas flow passage
freezing prevention flag is set off if the temperature of the gas
discharged from the jet pump 25 (discharged gas temperature)
exceeds the freezing release threshold value Th_e. The switcher 320
outputs zero as the value for releasing the gas temperature
increase request flow rate when the gas flow passage freezing
prevention flag is set off. Specifically, the switcher 320 limits
an increase of the flow rate of the cooling water to be supplied to
the heat exchanger 23 on the basis of the temperature of the gas
discharged from the jet pump 25.
[0238] Since the cooling water flow rate can be prevented from
being unnecessarily increased in this way, an increase of the power
consumption of the cooling water pump 32 can be suppressed.
Further, the warm-up of the fuel cell stack 1 can be promoted by
limiting an increase of the cooling water flow rate.
[0239] Further, in the present embodiment, the discharged gas
temperature calculation unit 120 calculates the discharged gas
temperature on the basis of the temperature of the gas supplied
from the heat exchanger 23 to the jet pump 25 and the temperature
of the gas to be circulated to jet pump 25.
[0240] When the discharged gas temperature increases to the
freezing release threshold value (limit threshold value) of e.g.
0.degree. C., the switcher 320 switches the flow rate of the
cooling water to be supplied to the heat exchanger 23 to the
warm-up request flow rate smaller than the gas temperature increase
request flow rate.
[0241] In this way, it is possible to promote the warm-up of the
fuel cell stack 1 while suppressing an unnecessary increase of the
cooling water flow rate. Further, since it is not necessary to
newly provide a block for limiting the cooling water flow rate, a
calculation load can be suppressed.
[0242] It should be noted that an example of increasing the flow
rate of the cooling water to be supplied to the heat exchanger 23
from the warm-up request flow rate when the circulating gas
temperature is not lower than the moisture increase threshold value
Th_s and the discharged gas temperature is not higher than the
freezing release threshold value Th_e has been described in the
present embodiment.
[0243] In such a case, the gas flow passage freezing prevention
control unit 230 may increase the flow rate of the cooling water to
be supplied to the heat exchanger 23 from the warm-up request flow
rate when the circulating gas temperature is not lower than the
freezing point and the discharged gas temperature is not higher
than the freezing point.
[0244] The moisture increase threshold value Th_s and the freezing
release threshold value Th_e are set at 0.degree. C. in this way
for the following reason. Firstly, no ice is generated in the jet
pump 25 unless the discharged gas temperature is not higher than
0.degree. C. Further, as shown in FIG. 9, the amount of steam
contained in the anode off-gas is very small even if the
post-joining gas temperature is not higher than the freezing point
unless the circulating gas temperature is higher than the 0.degree.
C. This, ice is hardly generated in the jet pump 25.
[0245] Accordingly, ice generated in the jet pump 25 increases when
the circulating gas temperature is not lower than 0.degree. C. and
the discharged gas temperature is not higher than 0.degree. C.
Thus, the cooling water flow rate is increased when such conditions
hold. In this way, the anode gas is heated by the heat exchanger 23
only in a state where ice increases in the jet pump 25, wherefore
the freezing of the jet pump 25 can be precisely prevented.
Third Embodiment
[0246] FIG. 11 is a diagram showing a configuration example of a
fuel cell system 101 in a third embodiment of the present
invention.
[0247] The fuel cell system 101 includes a bypass cooling water
pump 38 in addition to the configuration of the fuel cell system
100 shown in FIG. 1. In the following description, the same
components as in the fuel cell system 100 are denoted by the same
reference signs and not described.
[0248] The bypass cooling water pump 38 is provided in a branch
passage 37 located between a part where the branch passage 37 is
branched off from a cooling water circulation passage 31 and a heat
exchanger 23. The bypass cooling water pump 38 is controlled by a
controller 110.
[0249] The controller 110 increases a flow rate of cooling water
supplied from the bypass cooling water pump 38 to the heat
exchanger 23 if a gas flow passage freezing prevention flag is
switched on in Step S107 of FIG. 3.
[0250] FIG. 12 is a block diagram showing an example of the
configuration of a cooling water flow rate control unit 201
provided in the controller 110 in the present embodiment.
[0251] The cooling water flow rate control unit 201 includes an
adder 350, a cooling water target flow rate setting unit 360 and a
bypass target flow rate setting unit 370 instead of the request
flow rate setting unit 330 and the cooling water target flow rate
setting unit 340 shown in FIG. 5. Since the other components are
the same as those of the cooling water flow rate control unit 200
shown in FIG. 5, they are denoted by the same reference signs and
not described.
[0252] The adder 350 adds a gas temperature increase request flow
rate after correction or zero output from a switcher 320 to an
excessive temperature increase preventing flow rate. For example,
if the gas flow passage freezing prevention flag is set on, the
adder 350 outputs a value obtained by adding the gas temperature
increase request flow rate to the excessive temperature increase
preventing flow rate as a total cooling water flow rate to the
cooling water target flow rate setting unit 360.
[0253] The cooling water target flow rate setting unit 360 sets the
larger one of the total cooling water flow rate output from the
adder 350 and a normal control flow rate or warm-up request flow
rate output from a switcher 310 as a cooling water target flow
rate. Then, the cooling water target flow rate setting unit 360
outputs the cooling water target flow rate to each of the bypass
target flow rate setting unit 370 and a cooling water pump rotation
speed calculation unit 410.
[0254] For example, when the gas flow passage freezing prevention
flag is set on with a stack warm-up flag set on, the cooling water
target flow rate setting unit 360 outputs the total cooling water
flow rate output from the adder 350 as the cooling water target
flow rate.
[0255] Further, when the gas flow passage freezing prevention flag
is set off with the stack warm-up flag set on, the cooling water
target flow rate setting unit 360 outputs the excessive temperature
increase preventing flow rate output from the adder 350 as the
cooling water target flow rate.
[0256] The bypass target flow rate setting unit 370 outputs a value
obtained by subtracting the excessive temperature increase
preventing flow rate from the cooling water target flow rate as a
set value of the bypass target flow rate to the bypass cooling
water pump rotation speed calculation unit 420.
[0257] For example, when the gas flow passage freezing prevention
flag is set on with the stack warm-up flag set on, the bypass
target flow rate setting unit 370 outputs a value obtained by
subtracting the excessive temperature increase preventing flow rate
from the total cooling water flow rate, i.e. the gas temperature
increase request flow rate.
[0258] In this way, the cooling water is supplied to the fuel cell
stack 1 at a flow rate equivalent to the excessive temperature
increase preventing flow rate by the cooling water pump 32 and
supplied to the heat exchanger 23 at a flow rate equivalent to the
gas temperature increase request flow rate by the bypass cooling
water pump 38.
[0259] Further, when the gas flow passage freezing prevention flag
is set off with the stack warm-up flag set on, the bypass target
flow rate setting unit 370 outputs a value obtained by subtracting
the excessive temperature increase preventing flow rate from the
warm-up request flow rate.
[0260] In this way, the cooling water is supplied to the fuel cell
stack 1 at a flow rate equivalent to the warm-up request flow rate
by the cooling water pump 32 and supplied to the heat exchanger 23
at a flow rate equivalent to the excessive temperature increase
preventing flow rate by the bypass cooling water pump 38.
[0261] The cooling water pump rotation speed calculation unit 410
calculates a rotation speed of the cooling water pump 32 on the
basis of the cooling water target flow rate. Further, the cooling
water pump rotation speed calculation unit 410 corrects the
rotation speed of the cooling water pump 32 according to the
cooling water temperature of the fuel cell stack 1.
[0262] In the present embodiment, a rotation speed command map
indicating a relationship between the cooling water target flow
rate and the cooling water pump rotation speed for each cooling
water temperature is stored in the cooling water pump rotation
speed calculation unit 410. The rotation speed command map is
described later with reference to FIG. 13.
[0263] When obtaining the cooling water temperature and the cooling
water target flow rate, the cooling water pump rotation speed
calculation unit 410 refers to the rotation speed command map
specified by the cooling water temperature and calculates the
rotation speed associated with the cooling water target flow rate.
The cooling water pump rotation speed calculation unit 410 commands
that rotation speed to the cooling water pump 32.
[0264] The bypass cooling water pump rotation speed calculation
unit 420 calculates a rotation speed of the bypass cooling water
pump 38 on the basis of a bypass target flow rate. Further, the
bypass cooling water pump rotation speed calculation unit 420
corrects the rotation speed of the bypass cooling water pump 38
according to the rotation speed of the cooling water pump 32.
[0265] It should be noted that the bypass cooling water pump
rotation speed calculation unit 420 may increase the rotation speed
of the bypass cooling water pump 38 as the cooling water
temperature of the fuel cell stack 1 decreases since the cooling
water becomes less viscous.
[0266] In the present embodiment, a bypass rotation speed command
map indicating a relationship between the bypass target flow rate
and the bypass cooling water pump rotation speed for each rotation
speed of the cooling water pump 32 is stored in the bypass cooling
water pump rotation speed calculation unit 420. The bypass rotation
speed command map is described later with reference to FIG. 14.
[0267] When obtaining the bypass target flow rate and the rotation
speed of the cooling water pump 32, the bypass cooling water pump
rotation speed calculation unit 420 refers to the bypass rotation
speed command map specified by the rotation speed of the cooling
water pump 32 and calculates the rotation speed associated with the
bypass target flow rate. The bypass cooling water pump rotation
speed calculation unit 420 commands that rotation speed to the
bypass cooling water pump 38.
[0268] FIG. 13 is a graph showing an example of the rotation speed
command map stored in the cooling water pump rotation speed
calculation unit 410.
[0269] As shown in FIG. 13, the cooling water target flow rate and
the rotation speed of the cooling water pump 32 are associated with
each other for each cooling water temperature of the fuel cell
stack 1 in the rotation speed command map. In the rotation speed
command map, the rotation speed of the cooling water pump 32
nonlinearly increases as the cooling water target flow rate
increases. Further, at the same cooling water target flow rate, the
rotation speed of the cooling water pump 32 increases as the
cooling water temperature decreases since the cooling water becomes
less viscous.
[0270] FIG. 14 is a graph showing an example of the bypass rotation
speed command map stored in the bypass cooling water pump rotation
speed calculation unit 420.
[0271] As shown in FIG. 14, the bypass target flow rate and the
rotation speed of the bypass cooling water pump 38 are associated
with each other for each rotation speed of the cooling water pump
32 in the bypass rotation speed command map.
[0272] In the bypass rotation speed command map, the rotation speed
of the bypass cooling water pump 38 nonlinearly increases as the
bypass target flow rate increases. Further, at the same bypass
target flow rate, the rotation speed of the bypass cooling water
pump 38 increases as the rotation speed of the cooling water pump
32 decreases since the cooling water becomes less likely to flow
into the heat exchanger 23 via the bypass cooling water pump
38.
[0273] According to the third embodiment of the present invention,
the bypass cooling water pump 38 is provided in the branch passage
37 branched off from the cooling water circulation passage 31. The
cooling water flow rate control unit 201 increases the rotation
speed of the bypass cooling water pump 38 to increase the flow rate
of the cooling water to be supplied to the heat exchanger 23 to be
more than the warm-up request flow rate of the cooling water to be
supplied to the fuel cell stack 1 when the gas flow passage
freezing prevention flag is set on.
[0274] Since the temperature of the anode gas to be supplied to the
jet pump 25 increases and the temperature of the anode gas
discharged from the jet pump 25 increases to the freezing point in
a short time in this way, the closure of the flow passage by ice
generated in the jet pump 25 can be prevented.
Fourth Embodiment
[0275] FIG. 15 is a diagram showing a configuration example of a
fuel cell system 102 in a fourth embodiment of the present
invention.
[0276] The fuel cell system 102 includes a bypass valve 39 instead
of the bypass cooling water pump 38 of the fuel cell system 101
shown in FIG. 11. In the following description, the same components
as in the fuel cell system 101 are denoted by the same reference
signs and not described.
[0277] The bypass valve 39 is a three-way valve provided in a part
where a branch passage 37 is branched off from a cooling water
circulation passage 31. The bypass valve 39 is controlled by a
controller 110.
[0278] The controller 110 increases a flow rate of cooling water
supplied from the bypass valve 39 to a heat exchanger 23 if a gas
flow passage freezing prevention flag is switched from off to on in
Step S107 of FIG. 3.
[0279] FIG. 16 is a block diagram showing an example of the
configuration of a cooling water flow rate control unit 202
provided in the controller 110 in the present embodiment.
[0280] The cooling water flow rate control unit 202 includes a
bypass valve opening degree calculation unit 430 instead of the
bypass cooling water pump rotation speed calculation unit 420 shown
in FIG. 12. Since the other components are the same as those of the
cooling water flow rate control unit 200 shown in FIG. 5, they are
denoted by the same reference signs and not described.
[0281] The bypass valve opening degree calculation unit 430
calculates an opening degree of the bypass valve 39 on the basis of
a bypass target flow rate. Further, the bypass valve opening degree
calculation unit 430 corrects the opening degree of the bypass
valve 39 according to the cooling water temperature of a fuel cell
stack 1. Further, the bypass valve opening degree calculation unit
430 may reduce the opening degree of the bypass valve 39 as a
rotation speed of a cooling water 32 increases.
[0282] In the present embodiment, a bypass opening degree command
map indicating a relationship between the bypass target flow rate
and the opening degree of the bypass valve 39 for each cooling
water temperature of the fuel cell stack 1 is stored in advance in
the bypass valve opening degree calculation unit 430.
[0283] FIG. 17 is a graph showing an example of the bypass opening
degree command map stored in the bypass valve opening degree
calculation unit 430. Here, the flow rate of the cooling water to
be supplied to the heat exchanger 23 is increased by opening the
bypass valve 39 as the opening degree of the bypass valve 39
increases.
[0284] As shown on FIG. 17, the bypass target flow rate and the
opening degree of the bypass valve 39 are associated with each
other for each rotation speed of the cooling water pump 32 in the
bypass opening degree command map.
[0285] In the bypass opening degree command map, the opening degree
of the bypass valve 39 nonlinearly increases as the bypass target
flow rate increases. Further, at the same bypass target flow rate,
the opening degree of the bypass valve 39 increases as the rotation
speed of the cooling water 32 decreases since the cooling water
becomes less likely to flow into the heat exchanger 23.
[0286] When obtaining the bypass target flow rate and the rotation
speed of the cooling water pump 32, the bypass valve opening degree
calculation unit 430 refers to the bypass opening degree command
map specified by the rotation speed of the cooling water pump 32
and calculates the opening degree associated with the bypass target
flow rate. Then, the bypass valve opening degree calculation unit
430 commands that opening degree to the bypass valve 39.
[0287] According to the fourth embodiment of the present invention,
the bypass valve 39 is provided in the branch passage 37 branched
off from the cooling water circulation passage 31. The cooling
water flow rate control unit 202 opens the bypass valve 39 to
increase the flow rate of the cooling water to be supplied to the
heat exchanger 23 to be more than the warm-up request flow rate of
the cooling water to be supplied to the fuel cell stack 1 when the
gas flow passage freezing prevention flag is set on.
[0288] Since the temperature of the anode gas to be supplied to the
jet pump 25 increases and the temperature of the anode gas
discharged from the jet pump 25 increases to the freezing point in
a short time in this way, the closure of the flow passage by ice
generated in the jet pump 25 can be prevented.
[0289] Although the embodiments of the present invention have been
described above, the above embodiments are merely an illustration
of some application examples of the present invention and not
intended to limit the technical scope of the present invention to
the specific configurations of the above embodiments.
[0290] It should be noted that the above embodiments can be
combined as appropriate.
[0291] The present application claims the benefit of priority from
Japanese Patent Application No. 2014-151268, filed in the Japan
Patent Office on Jul. 24, 2014, the disclosure of which is
incorporated herein by reference in its entirety.
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